Overall Program Project Grant: Project Summary/Abstract Hematopoietic cell transplantation (HCT) is an effective therapy for a broad range of hematological malignances and represents the first successful, widely applied cellular therapy for cancer. The mechanisms of improved outcomes are focused on hematopoietic and immunological engraftment from the donor to the recipient, resulting in graft vs tumor effects and immunological reconstitution. However, several major challenges remain, including immunological recognition and attack of the host termed graft vs host disease (GVHD), immunological incompetence resulting in frequent opportunistic infections, reactivation of endogenous viruses and disease relapse. We have made excellent progress in the prior funding period as detailed in the application. The overall goals of our Program Project Grant are to develop a more robust fundamental understanding of transplantation biology and address the major challenges of HCT including GVHD, disease relapse and immune reconstitution. We will tackle these major challenges through innovative animal modeling, comprehensive biological and molecular analysis, novel imaging of animal and human subjects and biologically focused translational clinical trials. In addition, we will explore the biology of key cellular populations including Treg, iNKT, TR1, CD8 memory and CAR T cells. Our Program involves 5 interactive Projects focusing on the biology and prevention of GVHD through an enhanced understanding of cellular imaging and immune regulation (Projects 1 and 2), prevention and treatment of disease relapse with memory CD8+ T cells and vaccination strategies (Project 3), improved function and efficacy of CAR T cells (Project 4) and monoclonal antibody based strategies (Project 5). These Projects are supported by three Cores (Administration and Clinical Trial Coordination, Biostatistics and Data Management and Cell Processing and Immune Monitoring). Through our interactive Program Project, we will gain novel insights into the biology of hematopoietic cell transplantation and develop innovative strategies to improve outcomes for patients with hematological malignancies. The knowledge gained has profound implications for cancer and transplantation biology and also for the treatment of patients with a broad range of immunological conditions such as autoimmune disorders and transplantation of solid organs.